A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In an immersion lithographic apparatus, immersion fluid is handled by a fluid handling system or apparatus. In an embodiment the fluid handling system or apparatus may supply immersion fluid and therefore comprise or consist of a fluid supply system or apparatus. In an embodiment the fluid handling system or apparatus may at least partly confine immersion fluid. In an embodiment the fluid handling system or apparatus may provide a barrier to immersion fluid and thereby comprise or consist of a barrier member, such as a fluid confinement structure. In an embodiment the fluid handling system or apparatus may create or use a flow of gas, for example to help in controlling the flow and/or the position of the immersion fluid. The flow of gas may form a seal to confine the immersion fluid so the fluid handling system or apparatus may be referred to as a seal member; such a seal member may be a fluid confinement structure. In an embodiment, immersion liquid is used as the immersion fluid. In that case the fluid handling system or apparatus may be a liquid handling system or apparatus. In the following description, reference to a feature defined with respect to fluid may be understood to include a feature defined with respect to liquid.
The substrate may be supported by a support table during a lithographic process. A substrate handler may be provided for transferring the substrate to and/or from the support table. A vacuum clamping system may be used to clamp the substrate to the support table. In one arrangement, the substrate may be transferred from the substrate handler onto support pins. The support pins are configured to descend slowly until the substrate is transferred from the support pins onto the support table. The substrate may be transferred from the support pins onto burls protruding from a base surface of the support table, for example. The support pins are sometimes referred to as E-pins. While the substrate is being lowered onto the support pins, gas may be pumped away from a region beneath the substrate. This pumping away of gas may create a partial vacuum beneath the substrate. A non-uniform pressure profile may build up as the substrate approaches the support table. The non-uniform pressure profile may cause internal stresses to be imparted to the substrate. When the substrate is brought into contact with the support table, a partial seal is created and the pressure beneath the substrate falls quickly. The rapidly falling pressure results in the substrate being clamped quickly and firmly to the support table. Once clamped, subsequent movement of the substrate is restricted. Internal stresses existing in the substrate prior to clamping may persist after clamping and cause subsequent deformation of the substrate.
Clamping of substrates with internal stresses may lead to overlay error. It is estimated that the internal stresses may typically contribute about 1 nm of overlay error for substrates that are initially flat. The internal stresses will tend to contribute even more overlay error for substrates that are not initially flat.
For substrates that are initially flat the internal stresses will tend to cause deformation of the substrates after clamping. For substrates which are initially non-flat the internal stresses will tend to cause an increase in the deformation after clamping. The overlay error will tend to be larger for the substrates that are initially non-flat because both the initial non-flatness and the non-uniform pressure induced stresses will tend to contribute cumulatively to a larger overall deformation after clamping.
It is therefore desirable, for example, to alleviate one or more of the aforementioned problems or one or more other problems, by reducing the extent to which substrates are deformed after they have been loaded onto the support table.